1. Field of the Invention
The present invention relates to an improvement of circuit construction method of an active filter having a low pass notch filter characteristic.
2. Description of the Related Art
In an active filter there are filters of various types and well used one thereof is a circuit of type so called Delyiannis-Friend. In this circuit there are a band-pass filter, various notch filters, all-pass filter and etc. A low pass notch filter thereof is adopted herein.
Firstly, this filter will be explained and since for example, in page 401 of xe2x80x9cANALOG FILTER DESIGNxe2x80x9d (translators: Tatehiro Yanagisawa and Gen Kanai, original writer: M. E. Vanvalkenburg) details thereof is explained, that explanation is done by use of the result.
According to said document, said low pass notch filter has a circuit construction shown in FIG. 2(a) wherein by determining a center angle frequency xcfx89o, a notch angle frequency xcfx89z and a quality factor Q, each of element values can be represented by using these parameters as shown in FIG. 2(b). Here, R1 to R5 are resistors, C is a capacitor, OP is an operational amplifier, IN is a signal input terminal, OUT is a signal output terminal. These element values are normalized with the resistor of 1 xcexa9, the capacitor of 1 F, the angle frequency xcfx89o of 1 and the notch angle frequency xcfx89z is normalized with xcfx89o.
Accordingly, if for example, Q is 5 and xcfx89z is 1.1 herein, each of the element values are as shown in FIG. 2(c) and if a resistor scaling factor Ro=1Kxcexa9 and the angle frequency scaling factor xcfx89o=2xcfx80xc3x971000 Hz, finally, a circuit shown in FIG. 2(d). In this case, it can be understood that a minimum resistor value R3 is 24.2 xcexa9 and a maximum resistor value R5 is 476.19Kxcexa9 from this figure and 24.2 xcexa9 of the minimum resistor value can be omitted when range of resistors is considered because this resistor belongs to only a voltage dividing circuit. Thus, a minimum resistor value R1 is 1 kxcexa9. The maximum resistor value of 476.19Kxcexa9 becomes larger as Q becomes larger and xcfx89z becomes close to 1. However, when this resistor value becomes higher, an operational amplifier having an input impedance higher than that of said amplifier is required and it results in problems of limit of pass band and etc. to make this impedance higher. So, as shown in FIG. 2(e), it is considered that all resistors are decreased inversely in spite of increasing capacity of the capacitor C but when said resistors are decreased so much, since there is a strong possibility of xcfx89o changing depending on variation of an output impedance of a circuit for driving said circuit, this is not desirable. Therefore, if according to the circuit construction of FIG. 2(a) it is wanted to improve the above problem, it has to employ such measure that Q is not increased so much and xcfx89z is not made close to 1 so much, or to obtain necessary characteristic the number of circuit steps is increased.
The object of the present invention is to provide a circuit construction method of an active filter capable of obtaining a desired low pass notch filter even if Q is large and xcfx89z is close to 1, which can not be realized by the above described conventional circuit construction, in order to solve the above problem of an active filter, in particular an active filter having a low pass notch filter characteristic.
In order to attain the above object, according to a circuit construction method of an active filter of the present invention, an active filter having a low pass notch filter is constructed by using an operational amplifier, two capacitors and three voltage dividing circuits based on the following idea.
For convenience""s sake of explanation, said two capacitors are numbered. That is, a capacitor connected between a signal input circuit and an output circuit of the operational amplifier is C1 and a capacitor connected between the signal input circuit and an inversed input terminal of the operational amplifier is C2. Further, the following counterplanes are employed together.
(1) The circuit is so constructed that a signal to the inversed input terminal of the operational amplifier is seem of a signal due to the voltage dividing circuit from the output of the operational amplifier and a signal charged/discharged through the capacitor C2 from the signal input circuit, but this is changed to such circuit construction that a divided voltage of the voltage dividing circuit is charged/discharged to/from the capacitor C2 through a charge/discharge resistor.
(2) A signal attenuator (a voltage dividing circuit) inserted between connection points of the signal input terminal and the two capacitors.
(3) An attenuation quantity in the voltage dividing circuit between the signal input terminal and a noninversed input terminal of the operational amplifier is increased by an attenuation quantity due to (2).
(4) Since a gain as a filter circuit reduces according to said (2) and (3), dividing loss due to the voltage dividing circuit is increased by that reduction quantity and at the same time feed back quantity from an output of the operational amplifier to the capacitor C1 is decreased by the same quantity in order to recover reduction of circuit gain.
An active filter of the present invention based on the above (1) to (4) comprises first, second and third voltage dividing circuits, an operational amplifier, first and second capacitors, a charge/discharge resistor. An output terminal the operational amplifier is connected to an inversed input terminal of the operational amplifier through the second voltage dividing circuit and the charge/discharge resistor. The first capacitor is connected between the first voltage dividing circuit and the second voltage dividing circuit. The second capacitor is connected between the first voltage dividing circuit and the inversed input terminal of the operational amplifier. The third voltage dividing circuit is connected to the noninversed input terminal of the operational amplifier. The signal input terminal is connected to the first and the third voltage dividing circuit, and the signal output terminal is connected to an output terminal of the operational amplifier.
In the above active filter of the present invention, the following construction may be employed in order to obtain a low pass notch filter characteristic. A driving voltage to the inversed input terminal of the operational amplifier is formed by applying the divided voltage of the second voltage dividing circuit to the second capacitor. A signal from an output terminal of the operational amplifier to the second voltage dividing circuit is attenuated further by attenuation quantity equal to that due to the first voltage dividing circuit and by feeding back this attenuated signal to a divided output point of the first voltage dividing circuit through the first capacitor reduced quantity of the circuit gain due to the first voltage dividing circuit is recovered. At the same time, attenuation quantity of the third voltage dividing circuit is reduced further by equal quantity.